The present invention relates generally to an optical moisture sensor for detecting moisture on the surface of a transparent material, and more particularly, to a moisture sensor digital signal processing filtering for reducing the effects of unwanted signal components.
The accumulation of moisture on transparent materials, such as glass or Plexiglass, can obstruct a person's view through the material. Motor vehicles have long been equipped with motor-driven windshield wipers for clearing the moisture from the external surface of the windshield, at least within the driver's field of vision, and generally over a larger area so as to enhance one's vision through the windshield.
In most vehicles today, the windshield wiper system includes multi-position or variable speed switches which allow the driver to select a wide, if not an infinitely variable, range of speeds to suit conditions. Wiper controls are manually operated and typically include a delay feature whereby the wipers operate intermittently at selected time delay intervals.
Wiper control systems have recently been developed which include a moisture sensor mounted on one of the vehicle windows to automatically activate the wiper motor when moisture is deposited upon the surface of the window. The wiper control system including the moisture sensor are most typically mounted on the windshield, although the system may be mounted on the rear window or any other glass surface intended to be cleared of moisture. Such wiper control systems free the driver from the inconvenience of frequently adjusting the wiper speed as the driving conditions change.
Wiper control systems have used a number of different technologies to sense the moisture conditions encountered by a vehicle, including conductive, capacitive, piezoelectric, and optical sensors. Optical sensors operate upon the principle that a light beam is diffused or deflected from its normal path by the presence of moisture on the exterior surface of the windshield. The systems which employ optical sensors have the singular advantage that the means of sensing disturbances in an optical path is directly related to the phenomena observed by the driver (i.e., disturbances in the optical path that affords the driver vision). McCumber et al. (U.S. Pat. No. 4,620,141) disclose an optical moisture sensor which triggers a sweep of the wiper blades in response to the presence of water droplets on the exterior surface of a windshield.
In optical moisture sensors, a pulsatile light signal from an emitter is directed into the windshield and reflected back by the outer surface of the windshield and into a detector. The presence of moisture on the surface of the windshield affects the reflection of the pulsatile light signal at the outer surface of the windshield resulting in a reflected signal having a lower amplitude. The detector receives the reflected pulsatile signal and produces a pulsatile output signal which indicates the change in amplitude of the reflected emitter signal. The detector output signal also contains unwanted signals such as those from ambient light, electromagnetic interference, and white noise generated by the detector. These undesirable signal components must be removed before accurate moisture readings can be obtained.
It is known to use filters to remove unwanted signal components from the pulsatile detector signal. Schierbeek et al. in U.S. Pat. No. 4,916,374 show that a photovoltaic cell may be connected to a current to voltage amplifier, a high pass filter, and a further voltage amplifier, and subsequently demodulated using a sample and hold. The filters and amplifiers are of '374 are thus pre-demodulation circuits which act on the pulsatile signal before a demodulator converts it into a dc signal that is affected by sensed moisture. In U.S. Pat. No. 4,956,591, Schierbeek at al. use a high pass filter to remove the DC bias, and amplification located before demodulation.
However, the use of wide band pre-demodulation gain increases the amplitude of broadband unwanted noise especially white noise caused by random recombination of electrons and holes in the photodiodes. Such noise is inherent in the devices. It is desirable to use wideband pre-demodulation gain to increase the signal strength for improved sensitivity to moisture events. Without such gain, the sensor may not respond to the effects of small moisture droplets. Wideband pre-demodulation gain, however, amplifies the unwanted noise along with the desired signal. It is therefore, desirable to remove unwanted noise in the moisture detector signal after demodulation so that the sensor may be able to respond to small droplets of moisture.